Silicon Carbide on Diamond Substrates and Related Devices and Methods

ABSTRACT

A high power, wide-bandgap device is disclosed that exhibits reduced junction temperature and higher power density during operation and improved reliability at a rated power density. The device includes a diamond substrate for providing a heat sink with a thermal conductivity greater than silicon carbide, a single crystal silicon carbide layer on the diamond substrate for providing a supporting crystal lattice match for wide-bandgap material structures that is better than the crystal lattice match of diamond, and a Group III nitride heterostructure on the single crystal silicon carbide layer for providing device characteristics.

RELATED APPLICATIONS

This is a divisional application of copending application Ser. No.11/347,953 filed Feb. 6, 2006 and now U.S. Pat. No. ______. Ser. No.11/347,953 is a divisional of Ser. No. 10/707,898 filed Jan. 22, 2004,and now U.S. Pat. No. 7,033,912.

BACKGROUND

The present invention relates to semiconductor devices formed ofmaterials that make them suitable for high power, high temperature, andhigh frequency applications. As known to those familiar withsemiconductors, materials such as silicon (Si) and gallium arsenide(GaAs) have found wide application in semiconductor devices for lowerpower and (in the case of Si) lower frequency applications. Thesesemiconductor materials have failed to penetrate higher power highfrequency applications to the extent desirable, however, because oftheir relatively small bandgaps (e.g., 1.12 eV for Si and 1.42 for GaAsat room temperature) and relatively small breakdown voltages.

Accordingly, interest in high power high temperature and high frequencyapplications and devices has turned to wide bandgap semiconductormaterials such as silicon carbide (2.996 eV for alpha SiC at roomtemperature) and the Group III nitrides (e.g., 3.36 eV for galliumnitride at room temperature). These materials have higher electric fieldbreakdown strengths and higher electron saturation velocities ascompared to gallium arsenide and silicon.

Because of their wide and direct bandgap characteristics, the Group IIInitride semiconductors are candidates for many applications includingsolar blind photodetectors, blue light emitting and laser diodes, andhigh temperature and high power electronics. The galliumnitride-aluminum gallium nitride (GaN/AlGaN) heterostructure hasattracted special interest because of its potential applications in highmobility transistors operating at high powers and high temperatures.

In addition to other advantages, gallium nitride transistors cantheoretically or in actuality demonstrate several times the powerdensity as compared to gallium arsenide. Such higher power densitypermits smaller chips to handle the same amount of power which in turnprovides the opportunity to reduce chip size and increasing number ofchips per wafer, and thus lower the cost per chip. Alternatively,similarly sized devices can handle higher power thus providing sizereduction advantages where such are desirable or necessary.

As an exemplary driving force for higher-frequency, higher-powerdevices, cellular telephone equipment is rapidly becoming a large marketfor semiconductors, potentially exceeding even that of personalcomputers. This increase is driving a corresponding demand for thesupporting infrastructures to provide greater capabilities andperformance. Expected changes include the use of higher and higherfrequencies for obtaining appropriate spectrum space; e.g., from 900 MHzto higher frequencies including 2.1 GHz. Such higher frequency signalsaccordingly require higher power levels.

A high frequency high power device of particular interest is the highelectron mobility transistor (HEMT), and related devices such as amodulation doped field effect transistor (MODFET), or a heterojunctionfield effect transistor (HFET). These devices offer operationaladvantages under a number of circumstances because a two-dimensionalelectron gas (2DEG) is formed at the heterojunction of two differentsemiconductor materials with different bandgap energies, and where thesmaller bandgap material has a higher electron affinity. The 2DEG is anaccumulation layer in the undoped, smaller bandgap material and cancontain a very high sheet electron concentration on the order of 1012 to1013 carriers per square centimeter (cm-2). Additionally, electrons thatoriginate in the doped, wider-bandgap material transfer to the 2DEG,allowing a high electron mobility due to reduced ionized impurityscattering. In exemplary Group III nitride HEMTs, the two dimensionalelectron gas resides at the interface of a gallium nitride/aluminumgallium nitride heterostructure.

This combination of high carrier concentration and high carrier mobilitygives the HEMT a very large transconductance and a strong performanceadvantage over metal-semiconductor field effect transistors (MESFETs)for high-frequency applications. High electron mobility transistorsfabricated in the gallium nitride/aluminum gallium nitride (GaN/AlGaN)material system have the potential to generate large amounts of RF powerbecause of their unique combination of material characteristics whichincludes the aforementioned high breakdown fields, wide bandgaps, largeconduction band offset, and high saturated electron drift velocity.

Descriptions of recent progress in this field include, but are notlimited to, U.S. Pat. Nos. 6,586,781; 6,548,333; and 6,316,793; andpublished applications Nos. 20020167023 and 20030102482, the contents ofeach of which are incorporated entirely herein by reference. Relatedpublications include Pribble et al., Applications of SiC MESFETs and GaNHEMTs in Power Amplifier Design, International Microwave SymposiumDigest, 30:1819-1822 (2002).

High power semiconducting devices of this type operate in a microwavefrequency range and are used for RF communication networks and radarapplications and, as noted above, offer the potential to greatly reducethe complexity and thus the cost of cellular phone base stationtransmitters. Other potential applications for high power microwavesemiconductor devices include replacing the relatively costly tubes andtransformers in conventional microwave ovens, increasing the lifetime ofsatellite transmitters, and improving the efficiency of personalcommunication system base station transmitters.

As the output power and operational frequency of these devices continueto improve, the corresponding amount of heat generated from the device,and in turn from multi-device chips and circuits, has been and willcontinue to increase. Additionally, the design and market-generatedgoals for such devices include a continued reduction in size and weightof such electronic components. Therefore, packaging density hasincreased and will continue to increase. As a result, some accommodationmust be included to carry off excess heat or to otherwise moderate theeffects of heat on operating devices.

Excessive heat can raise several problems. Conductivity decreases athigher temperatures while maximum frequency and maximum power bothdecrease. Higher temperatures also permit more tunneling and leakingthat reduce device performance, and accelerate degradation and devicefailure. Stated more positively, improved thermal management can providehigher frequency operation and higher power density during a rateddevice lifetime.

For several crystal growth-related reasons, bulk (i.e., reasonably largesize) single crystals of Group III nitrides are, for practical purposes,unavailable. Accordingly, Group III nitride devices are typically formedon other bulk substrate materials, most commonly sapphire (Al2O3) andsilicon carbide (SiC). Sapphire is relatively inexpensive and widelyavailable, but is a poor thermal conductor and therefore unsuitable forhigh-power operation. Additionally, in some devices, conductivesubstrates are preferred and sapphire lacks the capability of beingconductively doped.

Silicon carbide has a better thermal conductivity than sapphire, abetter lattice match with Group III nitrides (and thus encourages higherquality epilayers), and can be conductively doped, but is also much moreexpensive. Furthermore, although progress has been made in designing anddemonstrating GaN/AlGaN HEMTs on silicon carbide (e.g., the patents andpublished applications cited above) a lack of consistent reliability atdesired rated performance parameters continues to limit commercialdevelopment.

Accordingly, the need exists for continued improvement in high frequencyhigh power semiconductor based microwave devices.

SUMMARY

In one aspect, the invention is a method of forming a high-power,high-frequency device in wide bandgap semiconductor materials withreduced junction temperature, higher power density during operation orimproved reliability at a rated power density or any combination ofthese advantages. In this aspect, the invention comprises adding a layerof diamond to a silicon carbide wafer to increase the thermalconductivity of the resulting composite wafer, thereafter reducing thethickness of the silicon carbide portion of the composite wafer whileretaining sufficient thickness of silicon carbide to support epitaxialgrowth thereon, preparing the silicon carbide surface of the compositewafer for epitaxial growth thereon, and adding a Group III nitrideheterostructure to the prepared silicon carbide face of the wafer.

In another aspect, the invention is a high-power, wide-bandgap devicethat exhibits reduced junction temperature and higher power densityduring operation and improved reliability at a rated power density. Inthis aspect, the invention comprises a diamond substrate for providing aheat sink with a thermal conductivity greater than silicon carbide, asingle crystal silicon carbide layer on the diamond substrate forproviding a supporting crystal lattice match for wide-bandgap materialstructures that is better than the crystal lattice match of diamond, anda Group III nitride heterostructure on the single crystal siliconcarbide layer for providing device characteristics.

In yet another aspect, the invention is a wide bandgap high electronmobility transistor (HEMT) comprising a diamond substrate for providinga heat sink with a thermal conductivity greater than that of anequivalent amount of silicon carbide, a semi-insulating silicon carbidesingle crystal layer on the diamond substrate for providing a favorablecrystal growth surface for Group III nitride epilayers (the terms“epitaxial layer” and “epilayer” are used interchangeably herein), afirst epitaxial layer of a first Group III nitride on the siliconcarbide substrate, a second epitaxial layer of a different Group IIInitride on the first epitaxial layer for forming a heterojunction withsaid first epilayer, and with the Group III nitride of the secondepilayer having a wider bandgap than the first Group III nitride of thefirst epilayer for generating a two dimensional electron gas (2DEG) inthe first epilayer at the interface of the first and second epilayers,and respective source and drain contacts to the second epitaxial GroupIII nitride epilayer for providing a flow of electrons between thesource and drain that is controlled by a voltage applied to the gate.

In yet another aspect, the invention is a wafer precursor forsemiconductor devices comprising a substrate of single crystal siliconcarbide that is at least two inches in diameter, a layer of diamond on afirst face of the silicon carbide substrate, and a second face that isprepared for growth of a Group III nitride epilayer or active structure.

In yet another aspect, the invention is a semiconductor laser comprisinga diamond substrate, a single crystal silicon carbide layer on thediamond substrate, at least a first cladding layer on the siliconcarbide layer, a Group III nitride active portion, and at least a secondcladding layer on the active portion.

The foregoing and other objects and advantages of the invention and themanner in which the same are accomplished will become clearer based onthe followed detailed description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a sequence of stepsillustrating the method of the invention.

FIG. 2 is a cross-sectional schematic diagram of a device structureaccording to the present invention.

FIG. 3 is another cross-sectional view of another device according tothe present invention.

FIG. 4 is a cross-sectional view of a wafer precursor according to thepresent invention.

FIG. 5 is a schematic cross-sectional view of the major portions of alaser structure according to the present invention.

DETAILED DESCRIPTION

It will be understood that the advantages of the invention as describedherein are neither exclusive of one another nor are they necessarilycumulative. Thus, by reducing the junction temperature, higher powerdensity can be achieved at a given rated reliability, or the reliabilitycan be increased at a previously-available rated power density, or somecombination of these (and other) advantages can be achieved.

In a first embodiment, the invention is a method of forming ahigh-powered device in wide bandgap materials with reduced junctiontemperature, higher power density during operation, and improvedreliability at a rated power density. The method is illustratedschematically in FIG. 1 which includes sub-Figures (A) through (E). FIG.1 illustrates a silicon carbide wafer 10. In preferred embodiments andas is most useful for device structures, the silicon carbide wafer orsubstrate 10 is a single crystal layer that provides a highly suitablesubstrate for the Group III nitride epitaxial layers and resultingdevices that are one of the principal objects of this invention. Theterm “wafer” is used in a broad sense herein and is not limited tocommon shapes and sizes.

FIG. 1(C) illustrates the next step in the method of the invention whichis that of adding a layer of diamond 11 to the silicon carbide substrate10. The diamond portion 11 increases the thermal conductivity of theresulting composite wafer, because the thermal conductivity of diamond(20 watts per centimeter per degree Kelvin (W/cm-K)) is approximatelyfour times that of silicon carbide (4.9 W/cm-K). In preferredembodiments, the silicon carbide wafer is prepared for the diamonddeposition by a chemical-mechanical polishing step (CMP). This can helpimprove the surface morphology of the grown diamond, and is mostpreferably carried out on the C-face of the SiC.

FIG. 1(D) illustrates that the next step is that of reducing thethickness of the silicon carbide portion of the composite wafer that isnow broadly designated at 12. A sufficient thickness of silicon carbideis retained to support epitaxial growth on the silicon carbide whileretaining the desired lattice matching characteristics of siliconcarbide. Preferably, the thickness is reduced as much as possible; i.e.,minimized, while remaining sufficient to provide the desired latticematching for the nitride epilayers. As recognized by those of relevant(“ordinary”) skill in this art, the lattice constants of silicon carbideare not necessarily identical to those of the Group III nitrides, butare closer than those of diamond. Stated differently, and as notedabove, large size Group III nitride single crystal substrates arecurrently unavailable from a practical standpoint. Accordingly, someother material is typically used as the substrate, and for a number ofreasons, silicon carbide is preferred and indeed from a lattice constantmatching standpoint, is better than diamond. Thus, those familiar withthe growth of epitaxial layers of Group III nitrides on silicon carbidewill recognize that the lattice constants do not need to matchidentically, but need to be favorably close enough to encourage thedesired high quality single crystal growth.

FIG. 1(B) illustrates one of the methods for reducing the thickness ofthe silicon carbide portion. In a preferred embodiment the siliconcarbide substrate 10 is implanted with oxygen at a predetermined depthin the silicon carbide to form a layer of silicon dioxide (SiO2)indicated as the dotted line 13. In this technique, the diamond layer 11is added next as previously described with respect to FIG. 1(C). Afterthe diamond has been deposited, the thickness of the silicon carbideportion 10 is reduced by separating the silicon carbide at the silicondioxide layer. This results in the structure illustrated in FIG. 13.This technique is also referred to as “SIMOX” (“separation byimplantation of oxygen”) and can include a heating step to furtherencourage the production of SiO2 from the implanted oxygen.

Alternatively, the step of reducing the thickness of the silicon carbideportion can comprise the more conventional steps of lapping andpolishing. As is recognized by those familiar with this art, however,the SiC-reducing step (or steps) need to be consistent with theremainder of the device manufacturing steps. Thus, overly aggressivemechanical steps that would harm or defeat the purpose of the diamondlayer or of the resulting device are preferably avoided.

The use of ion implantation with other elements or ions (e.g. H+)followed by separation at the implanted material is becoming more wellunderstood in the art and has been referred to as the “Smart Cut”process which was first developed to obtain silicon-on-insulatormaterials. Background sources on this technique are available, with arecent discussion being set forth by Moriceau et al., “New LayerTransfers Obtained by the Smart Cut Process,” Journal Of ElectronicMaterials, Vol. 32, No. 8, pages 829-835 (2003) and Celler et al.,“Frontiers of Silicon-on-Insulator,” Journal of Applied Physics, Vol.93, No. 9, pages 4955ff (2003). It will be understood that thesereferences are included for exemplary rather than limiting purposes.

Alternatively, the separation can be carried out using a thermal stresstechnique in which the wafer, having been implanted and followingdiamond growth, is cooled at a rate sufficient to separate the wafer atthe implanted portion, but less than the cooling rate at which the waferwould shatter. This separation could also be carried out between diamondgrowth steps as may be desired or necessary.

In preferred embodiments, the diamond layer is added to the carbon faceof the silicon carbide wafer and the silicon face is reserved for thesteps of preparing the silicon carbide surface of the composite wafer 12for epitaxial growth followed by the step of adding a Group III nitrideheterostructure to the prepared silicon carbide face of the wafer. Thus,the heterostructure is added to the silicon face of the composite wafer.The step is illustrated in FIG. 1(E) in which the orientation of thediamond portion 11 and the reduced thickness silicon carbide 10 havebeen flipped between FIGS. 1(D) and (E) so that the diamond 11 becomesthe substrate with a layer of silicon carbide 10 upon it. FIG. 1(E) alsoillustrates a heterostructure formed of two different Group III nitridesthe top portion of which is designated at 14 and a lower portion at 15.FIG. 1 also illustrates that in most embodiments, a buffer layer 16 willbe included between the silicon carbide layer 10 and the heterostructurelayers 14 and 15. As typical, but not exclusive, examples, theheterostructure layers are formed of aluminum gallium nitride (the toplayer 14) and gallium nitride (the middle layer 15), with aluminumnitride being selected for the buffer layer 16. Appropriate bufferlayers, heterostructures, growth methods and other relevant informationare set forth for example in the patents noted earlier, as well as inU.S. Pat. Nos. 5,393,993; 5,210,051; and 5,523,589, which are commonlyassigned with the present invention, and all of which are incorporatedentirely herein by reference.

As an additional consideration, however, the various CVD source gasesand equipment used to produce the buffer (if any) and theheterostructure layers should be selected to be compatible with thediamond. Stated differently, the source gases, equipment and relateditems should be selected to avoid any undesired reaction with or effectupon the diamond.

If desired, the diamond can be annealed to increase its insulatingcharacteristics. The annealing can be carried out in a furnace or in adiamond deposition chamber prior to growth of the Group III nitrideepilayers, or in the epilayer reactor prior to epilayer growth, orduring epilayer growth. Improved results have been observed when ammonia(NH3), hydrogen (H2) and nitrogen (N2) are used as ambient gases duringthe anneal. The reasons for such improvement are as yet undetermined,and thus the inventors do not wish to be bound by any particular theoryon this point.

In some embodiments, the diamond layer can be bonded to the siliconcarbide and these techniques are generally well understood in the art.In brief summary, bonding typically comprises placing the desiredmaterials in contact with one another while applying pressure and heat.In more preferred embodiments, however, the diamond is deposited on thesilicon carbide by chemical vapor deposition. Chemical vapor depositionof diamond has become more widely commercially available in recent yearsand exemplary services and equipment are available from sources such asP1 Diamond Inc. of Santa Clara, Calif., or Delaware Diamond Knives(“DDK”) of Wilmington, Del. Because the diamond is included for itsthermal properties, and because the silicon carbide provides the crystallattice matching, the diamond can be deposited in polycrystalline form.Although single crystal diamond will provide somewhat better thermalmanagement benefits, it is, like all single crystals, generally moredifficult or complex to produce than polycrystalline material. Thus, theuse of polycrystalline diamond is somewhat more convenient to carry out.As well understood by those in the art, chemical vapor deposition ofdiamond is typically produced by energizing mixtures of hydrogen andhydrocarbon gases with heat or electrical energy in a depositionreactor. The energy, source materials, and related parameters can all beadjusted in a suitable or desired manner; e.g., www.p1diamond.com andwww.ddk.com. Whenever possible, isotopically pure diamond (i.e., all12C) is also preferred over the naturally occurring isotopedistribution, which contains about one percent of 13C.

Furthermore, the diamond/SiC interface should have minimal thermalresistance, and thus where processes such as bonding are used, any voidsbetween the diamond and the SiC should be minimized or eliminated.

The diamond is deposited to a thickness that is sufficient to supportthe added heterostructure while avoiding additional material that failsto provide further functional benefit. Stated differently, once asufficient amount of diamond has been included to provide the requiredor desired thermal characteristics and mechanical support for theparticular wafer or device, merely adding further diamond offers nofurther advantage or functional benefit. In preferred embodiments, anappropriate diamond layer has a thickness of between about 100 and 300microns (μm) for the type of heterostructure Group III nitride devicesfor which the invention is particularly suited.

Because, however, devices are typically manufactured in multiple stepprocesses, the invention can further comprise depositing two (or more)layers of diamond (or one layer of diamond and another layer of a secondmaterial) that differ in properties from one another. The purpose ofadding the additional layer (or layers) is to provide one that has thethermal conductivity characteristics for the final device, while havingadditional (even if temporary) layers present for manufacturing purposesthat can be removed later. Thus, in this aspect the method comprisesdepositing a layer of semi-insulating diamond on semi-insulating siliconcarbide to provide a semi-insulating substrate for high frequencydevices. Thereafter a second layer of diamond (or other material) isdeposited on the semi-insulating layer to provide additional mechanicalstability during wafer processing. The additional portion added formechanical stability does not necessarily need to be semi-insulatingbecause in this aspect the method comprises further processing the wafer(e.g., any appropriate or generic steps) with the second diamond (orother material) layer and thereafter removing portions (or all) of thesecond layer as the devices are finished. For example, the second layercan be added to provide mechanical stability during handling and anumber of the epitaxial growth steps that result in heterojunctiondevices, but can then be removed prior to the step of opening via holesthough the wafer or device.

The deposited second layer can also comprise another material selectedfor a complementary purpose. For example, a less expensive material canbe selected provided it does not otherwise interfere with the othermethod steps or resulting device operation. Alternatively, a materialcan be selected to be somewhat easier to remove; e.g. silicon dioxide,silicon nitride, polycrystalline aluminum nitride, or silicon carbide.

With the diamond layer in place, the method next comprises preparing theopposite silicon carbide surface of the composite wafer for epitaxialgrowth thereon, and then adding a Group III nitride epilayer—andtypically several layers including a heterojunction—to the prepared face(which will be the Si-face when the diamond has been deposited on theC-face). Preferably, the SiC surface is prepared by another CMP step.

FIGS. 2 and 3 illustrate devices that advantageously incorporate thepresent invention. Wherever possible, corresponding elements carry thesame reference numerals in FIGS. 2 and 3 as in FIG. 1. Accordingly, FIG.2 illustrates a high-power, high-frequency wide bandgap device broadlydesignated at 20 that exhibits reduced junction temperature and higherpower density during operation and improved reliability at a rated powerdensity. In this aspect, the invention comprises a diamond substrate 11for providing a heat sink with a thermal conductivity greater thansilicon carbide, and that will thus carry off more heat than anequivalent amount of SiC. A single crystal silicon carbide layer 10 ison the diamond substrate 11 for providing a crystal lattice match forwide bandgap material structures that is better than the crystal matchof diamond. A Group III nitride heterostructure indicated by thebrackets 21 is on the single crystal silicon carbide layer for providingdevice characteristics. FIG. 2 also illustrates the aluminum nitridebuffer layer 16 which is preferably incorporated into the structure ofthe invention. It will accordingly be understood that when layers arereferred to as being “on” one another, the term can encompass bothlayers that are directly in contact and those that are above one anotherwith intermediate layers in between.

In a manner analogous to FIG. 1(E), the heterostructure 21 is in itsmost basic format formed of two different layers of Group III nitridesuch as the aluminum gallium nitride layer 14 and the gallium nitridelayer 15. It will be understood, however, by those of relevant skill inthis art that FIGS. 2 and 3 are schematic in nature and that morecomplex structures can be incorporated as desired or necessary,including double heterojunctions, barrier HEMTs, multiple quantum wells(MQWs), and superlattice structures. It will likewise be understood thatwhen the singular term “layer” is used to described portions of a GroupIII nitride device, it can include multiple layers that are commonlyincluded in such devices for both manufacturing and operationalpurposes.

Additionally, those familiar with the Group III nitrides will recognizethat the ternary Compounds such as AlGaN are more descriptivelyexpressed as AlxGa1−xN where 0<x<1 and that the respective molefractions of aluminum and gallium (as expressed by x and 1−x) can beadjusted to provide desired or necessary properties. The tertiary GroupIII nitrides can be expressed in the same manner; e.g. InxAlyGa1−x−yNwhere 0<x+y<1. Layers, junctions and devices formed of any of thesematerials can take advantage of the benefits offered by the presentinvention.

These formulas can also be written in “greater than or equal to” format(e.g., 0≦x≦1) when the goal is to express a greater range ofpossibilities; e.g. AlxGax−1N where 0≦x≦1 could represent AlN or GaN orAlGaN depending on the value of x.

In preferred embodiments, the diamond substrate 11 can bepolycrystalline as this generally provides for less complex and lessdemanding manufacturing and thus helps increase manufacturing efficiencyand reduce manufacturing costs.

Because a number of high frequency devices require semi-insulatingsubstrates, in such embodiments the silicon carbide layer 10 issemi-insulating and if necessary depending upon the overallcharacteristics, the diamond substrate 11 may be semi-insulating aswell. The nature and use of semi-insulating silicon carbide is set forthin exemplary (but not limiting) fashion in commonly assigned U.S. Pat.Nos. 6,218,680; 6,403,982; 6,396,080; 6,639,247 and 6,507,046; thecontents of each of which are incorporated entirely herein by reference.

The silicon carbide typically has a polytype selected from the 3C, 4H,6H and 15R polytypes of silicon carbide as these are the most widelyavailable and suitable for electronic devices. Suitable substrates arecommercially available from Cree, Inc. of Durham, N.C. (www.cree.com).

By adding the ohmic contacts 24 and 26 and the Schottky contact 25 tothe heterostructure 21 (FIG. 2), a field effect transistor can beproduced. In particular, by controlling the composition and doping ofthe epitaxial layers that form the heterostructure 21 a high frequencyhigh electron mobility transistor (HEMT) can be produced.

Because the diamond substrate 11 of the device 20 provides a highthermal conductivity, the device can be advantageously packaged in(i.e., with or adjacent) a high thermal conductivity material. As usedherein, the term “package” is used in its usual sense to refer to thecontainer or structure in which an individual semiconductor device iscontained for purposes of incorporating it into a larger circuit or anend use device. Indeed, because the thermal expansion of the device willbe dominated by the thermal expansion coefficient of diamond, thepackage can further include or be formed of diamond as may be desired ornecessary to minimize or eliminate stress from materials that aremismatched in their thermal expansion and to take full advantage of thethermal conductivity of the device.

In this regard, the channels of HEMTs and related devices tend togenerate the most heat during operation, particularly in the gateregion. Thus the invention is particularly helpful in both spreadingheat from the channel and also acting as a heat sink.

FIG. 3 illustrates a high electron mobility transistor (HEMT) insomewhat more detail and broadly designated at 30. The transistor 30 isformed of a diamond substrate 11 for providing the heat sink (orspreader) as discussed above. A semi-insulating layer of silicon carbide10 is on the diamond substrate 11 for providing a favorable crystalgrowth surface for the Group III nitride epilayers. A buffer layer 16,typically or preferably formed of another Group III nitride such asaluminum nitride, is on the silicon carbide layer 10 and provides acrystal transition from the silicon carbide to the heterostructure. Theheterostructure is formed of a first epitaxial layer 15 on the buffer 16and a second epitaxial layer (or layers) indicated by the brackets 31 onthe first epitaxial layer 15 for forming a heterojunction between theepilayers 15 and 31. In this embodiment, the Group III nitride of thesecond epilayer 31 has a wider bandgap than the first Group III nitrideof the first epilayer 15 for generating a two-dimensional electron gas(2DEG) in the first epilayer at the interface of the first and secondepilayers 15 and 31. Respective source 34, gate 35 and drain 36 contactsprovide a flow of the electrons between the source 34 and the drain 36that is controlled by a voltage applied to the gate contact 35.

In certain embodiments, the first epitaxial layer 15 comprises galliumnitride and the second epitaxial layer 31 comprises aluminum galliumnitride. More preferably, the gallium nitride layer 15 is undoped andthe aluminum gallium nitride layer 31 is doped n-type, for example withsilicon (Si). As further illustrated in FIG. 3, the second epitaxiallayer 31 includes two, and preferably three layers of aluminum galliumnitride that are respectively illustrated at 40, 41 and 42. Thus, in themore preferred embodiments the second epitaxial layer 31 comprises adoped layer 41 of aluminum gallium nitride and an undoped layer 40 ofaluminum gallium nitride with the undoped layer 40 being adjacent theundoped layer 15 of gallium nitride. In yet a further embodiment,another undoped layer 42 of AlGaN is provided on top of the doped AlGaNlayer 41. This structure is set forth and described in U.S. Pat. No.6,583,454, which is commonly assigned with the present invention andwhich is incorporated entirely herein by reference. The use of anundoped layer of aluminum gallium nitride between the gallium nitridelayer and the doped aluminum gallium nitride layer is referred to as amodulation-doped heterostructure and increases the mobility of electronsin the two-dimensional electron gas by physically separating the dopantin the aluminum gallium nitride layer from the undoped gallium nitridelayer.

As set forth in U.S. Pat. No. 6,583,454, it has also been found that thetransistor 30 operates more efficiently when a passivation layer 43 isincluded above the heterostructure formed by the layers 15 and 31. Asfurther set forth in the '454 patent, the passivation material can besilicon dioxide (SiO2) or silicon nitride (Si3N4) provided that it hasthe relevant passivating characteristics.

As set forth in previously-incorporated published application No.20020167023, the transistor can also incorporate a barrier layer forreducing gate leakage in the resulting device; or other layers (e.g.layers that improve the performance of the contact to p-type materials)for desired purposes, all while taking advantage of the presentinvention.

As with respect to the embodiment shown in FIG. 2, the transistor 30 ofFIG. 3 can be packaged in a high thermal conductivity material,potentially including a diamond or diamond-containing package.

Although specifically discussed herein in terms of the high electronmobility transistor, the invention offers advantages in other devicesfor which improved thermal management offers advantages. Such otherdevices can include lasers, one of which is schematically illustrated inFIG. 5 at 45. The general theory and operation of semiconductor lasersis well understood by those of appropriate (“ordinary”) skill in thisart and need not be discussed in detail herein other than to note thatthe laser is typically formed of an active layer 46, bordered byrespective clad layers 47 and 50. The nature of the materials for theclad layers—particularly their bandgaps and refractive indices—arepreferably selected so that the active layer forms a potential wellbetween the clad layers. Additionally, the refractive indexes of theclad layers 47 and 50 are preferably such that they encourage the lightto remain confined in the active layer for resonance purposes. As in theprevious embodiments, the diamond substrate is illustrated at 11 and thesilicon carbide layer at 10 with an appropriate buffer layer 16 includedas necessary. Generally, but not necessarily exclusively, lasers andlight emitting diodes (LEDs) are more likely to include more complexstructures such as multiple quantum wells and superlattice structures.

FIG. 4 is a schematic view of a wafer precursor according to the presentinvention. The wafer precursor is broadly designated at 52 and isincluded herein because the availability of large single crystalsubstrate wafers in silicon carbide—i.e., those of 2-inch., 3-inch, or100 mm diameter (or equivalent metric sizes)—likewise provide thepreviously unavailable opportunity to, using the method of theinvention, obtain diamond layers of this size for the heat sink purposesdescribed herein. In this aspect, the invention is a substrate of singlecrystal silicon carbide 53 that is at least 2 inches in diameter. Alayer of diamond 54 is on one face of the silicon carbide substrate 53and a Group III nitride active structure designated by the brackets 55is on the opposite face of the wafer 53. The substrate silicon carbidewafer 53 is preferably at least about 2 inches (5 centimeters (cm)) indiameter, more preferably at least 3 inches (7.5 cm) in diameter, andmost preferably at least 4 inches (10 cm) in diameter. As set forth withrespect to the other embodiments of the invention, the Group III nitrideactive structure 55 includes at least one heterostructure and the waferprecursor 52 can also include an appropriate buffer layer between thesilicon carbide substrate and between the substrate and theheterostructure 55. For purposes of simplicity, the buffer layer it isnot separately illustrated in FIG. 4. As in the previous embodiments,the silicon carbide can have a polytype selected from the groupconsisting of the 3C, 4H, 6H, and 15R polytypes of silicon carbide.

In the most preferred embodiments, the wafer includes a plurality ofindividual active structures on the silicon carbide substrate.

The invention has been primarily described herein with respect to theuse of diamond as the heat sink material and silicon carbide as thesubstrate material. The invention can be understood in a broader aspect,however, in which a layer of higher thermal conductivity material isused in conjunction with a lower thermal conductivity material that hasa better crystal lattice match with the Group III nitrides to produce aresulting composite wafer. In this aspect, the higher thermalconductivity materials can include metals and semiconductors such asboron nitride (BN), particularly cubic boron nitride (“cBN”), while themore preferred crystal substrate material can be selected from the groupconsisting of silicon, gallium nitride, aluminum nitride, aluminumgallium nitride, magnesium oxide (MgO), magnesium aluminate (MgAl2O4),lithium gallate (LiGaO2), lithium aluminate (LiAlO2), zinc oxide (ZnO),nickel aluminate (NiAl2O4), and sapphire, with AlGaN being morepreferred. As in the previously-described embodiments, the presence ofthe higher thermal conductivity material as part of the substrate waferfavorably increases the thermal conductivity of the resulting compositewafer.

Cubic boron nitride (“cBN”) is expected to have advantages in terms ofits capability to be made semi-insulating, and for being processed forpurposes of the invention in a manner similar to diamond. Boron nitridealso has a higher thermal conductivity (13 W/cm-K) than silicon carbide.

For devices such as lasers and LEDs, a metal heat sink can beadvantageous for reflection and light-extraction purposes. Suitablecandidate metals can include Ni, Cr, Mn, W, Pt and relevant alloys ofthese metals. As noted above with respect to the diamond heat sink, theuse of any one or more of these metals or alloys should be consistentwith the overall manufacture, structure, and operation of the resultingdevice.

As in the previous embodiments, the method of forming a device in such acomposite wafer comprises reducing the thickness of the lower thermalconductivity portion of the composite wafer while retaining a sufficientthickness of the lower thermal conductivity portion to support thedesired epitaxial growth thereon. The lower thermal conductivity surfaceis also prepared for epitaxial growth after which an appropriate GroupIII nitride epilayer, and typically a heterostructure, is added to theprepared face of the lower thermal conductivity portion of the wafer.

In the drawings and specification there has been set forth a preferredembodiment of the invention, and although specific terms have beenemployed, they are used in a generic and descriptive sense only and notfor purposes of limitation, the scope of the invention being defined inthe claims.

1. A wide-bandgap high electron mobility transistor (HEMT) comprising: adiamond substrate; a semi-insulating single crystal silicon carbidelayer on said diamond substrate for providing a favorable crystal growthsurface for Group III nitride epilayers; a buffer layer on said siliconcarbide layer; a first epitaxial layer of a first Group III nitride onsaid buffer layer; a second epitaxial layer of a different Group IIInitride on said first epitaxial layer for forming a heterojunction withsaid first epilayer, and with said Group III nitride of said secondepilayer having a wider bandgap than said first Group III nitride ofsaid first epilayer; and respective source, gate, and drain contacts tosaid second epitaxial Group III nitride layer.
 2. A HEMT according toclaim 1 wherein said first epitaxial layer comprises gallium nitride andsaid second epitaxial layer comprises aluminum gallium nitride.
 3. AHEMT according to claim 2 wherein said gallium nitride layer is undopedand said aluminum gallium nitride layer is n-doped.
 4. A HEMT accordingto claim 1 wherein said first epitaxial layer comprises gallium nitrideand said second epitaxial layer comprises a doped layer of aluminumgallium nitride and an undoped layer of aluminum gallium nitride, withsaid undoped aluminum gallium nitride layer adjacent said undopedgallium nitride layer.
 5. A HEMT according to claim 1 wherein said firstepitaxial layer comprises gallium nitride and said second epitaxiallayer comprises a layer of aluminum gallium nitride and an undoped layerof aluminum nitride, with said undoped aluminum nitride layer adjacentsaid undoped gallium nitride layer.
 6. A HEMT according to claim 1packaged in a high thermal-conductivity material.
 7. A HEMT according toclaim 1 and further comprising a mechanical substrate for said diamondsubstrate for providing additional mechanical support to said HEMT.
 8. AHEMT according to claim 1 wherein said diamond substrate is formed of atleast two discrete diamond layers that differ from one another in theirproperties.
 9. A HEMT according to claim 1 wherein said silicon carbidelayer is bonded to said diamond substrate.
 10. A device according toclaim 1 packaged in a high-thermal conductivity material.
 11. A packageddevice according to claim 6 wherein said high-thermal conductivitymaterial comprises diamond.
 12. A wide-bandgap high electron mobilitytransistor (HEMT) comprising: a diamond substrate; a semi insulatingsingle crystal silicon carbide layer on said diamond substrate; a GroupIII nitride buffer layer on said silicon carbide substrate; an epitaxiallayer of gallium nitride on said buffer layer; an epitaxial layer ofaluminum gallium nitride on said gallium nitride epitaxial layer forforming a heterojunction with said gallium nitride epilayer with saidaluminum gallium nitride having a wider bandgap than said galliumnitride epitaxial layer; and respective source, gate, and drain contactsto the aluminum gallium nitride layer.
 13. A HEMT according to claim 12wherein said gallium nitride layer is undoped and said aluminum galliumnitride layer is n-doped.
 14. A HEMT according to claim 12 wherein saidbuffer layer comprises aluminum nitride.
 15. A HEMT according to claim12 wherein said aluminum gallium nitride layer is formed of a dopedlayer and an undoped layer, with the undoped aluminum gallium nitridelayer adjacent said undoped gallium nitride layer and said ohmiccontacts being to said doped layer.
 16. A HEMT according to claim 12wherein said aluminum gallium nitride layer is formed of a doped layerbetween two undoped layers with one of said undoped layers beingadjacent said undoped gallium nitride layer and the other of saidundoped layers being in contact with said respective ohmic contacts. 17.A HEMT according to claim 12 and further comprising a passivation layerabove said heterojunction.
 18. A HEMT according to claim 12 packaged ina high-thermal conductivity material.
 19. A packaged device according toclaim 18 wherein said high-thermal conductivity material comprisesdiamond.
 20. A wafer precursor for semiconductor devices comprising: asubstrate of single crystal silicon carbide that is at least two inchesin diameter; and a layer of diamond on a first face of said siliconcarbide substrate; wherein the opposite face of said silicon carbidesubstrate is prepared for Group III nitride epitaxial growth thereon.21. A wafer precursor according to claim 20 comprising a Group IIInitride active structure on said prepared opposite face.
 22. A waferprecursor according to claim 20 wherein said silicon carbide substrateis at least three inches in diameter.
 23. A wafer precursor according toclaim 20 wherein said silicon carbide substrate is at least 100 mm indiameter.
 24. A wafer precursor according to claim 21 wherein said GroupIII nitride active structure includes at least one heterostructure. 25.A wafer precursor according to claim 24 comprising a buffer layer onsaid silicon carbide substrate and between said substrate and saidheterostructure.
 26. A wafer precursor according to claim 20 whereinsaid silicon carbide has a polytype selected from the group consistingof the 3C, 4H, 6H and 15R polytypes of silicon carbide.
 27. A waferprecursor according to claim 20 comprising a plurality of individualactive structures on said prepared face of said silicon carbidesubstrate.
 28. A semiconductor laser comprising: a diamond substrate; asingle crystal silicon carbide layer on said diamond substrate; at leasta first cladding layer on said silicon carbide layer; a Group IIInitride active portion on said at least one cladding layer; and at leasta second cladding layer on said active portion.
 29. A semiconductorlaser according to claim 28 further comprising a buffer layer betweensaid silicon carbide layer and said first cladding layer.
 30. Asemiconductor laser according to claim 28 comprising a polycrystallinediamond substrate.
 31. A semiconductor laser according to claim 28wherein said silicon carbide has a polytype selected from the 3C, 4H, 6Hand 15R polytypes of silicon carbide.